1. Field of the Invention
The invention relates to a method and to a device for operating an SCR system in an internal combustion engine.
2. Related Art
Within the scope of strict regulations relating to the emissions of pollutants emitted by motor vehicles, an important measure is to keep low the emissions of pollutants that are produced during the combustion of the air/fuel mixture in the respective cylinders of the internal combustion engine.
A further measure is also to use exhaust gas post-treatment systems that convert the emissions of pollutants generated during the combustion process of the air/fuel mixture in the respective cylinder into non-damaging substances. For this purpose, exhaust gas catalytic converters are used that convert carbon monoxide, hydrocarbons and also nitrogen oxides into non-damaging substances. In particular in the context of diesel internal combustion engines, for example particle filters are used and also SCR (selective catalytic reduction) catalytic converters are used. Such SCR catalytic converters selectively reduce the nitrogen oxides. This means that, in particular, nitrogen oxides such as NO, NO2 are reduced, while undesired secondary reactions are avoided. Ammonia (NH3) is necessary for this reaction.
In the case of SCR systems comprising an SCR catalytic converter, in the field of motor vehicles a reducing agent is metered into the exhaust gas train. Odorless urea is used as the reducing agent since ammonia in its pure form has a high level of toxicity and is also flammable. An aqueous urea solution, which is also marketed by the brand name AdBlue® is made available as the reducing agent. The reducing agent is generally stored in a separate tank in the vehicle.
The consumption of the reducing agent is between 2 to approximately 8% of the fuel, in particular of the diesel fuel.
In order to reduce the nitrogen oxide emissions as much as possible it is important that the reducing agent be metered with the correct ratio. If the metering is too low, the efficiency of the reduction in the nitrogen oxide drops. If the portion of urea is too high, the ammonia which is formed therefrom may to a certain extent not react with the nitrogen oxides.
DE 10 2006 013 263 A1 discloses a method for determining the concentration of a component of a fluid provided for exhaust gas purification, in which method a concentration of the component in the fluid is determined by means of a measurement of the speed of sound in the fluid.
WO 2007/104779 A2 discloses a method for checking a composition of an ammonia solution, in which method a reference temperature profile is recorded and a test temperature profile is compared with the reference temperature profile.